(1) Field of the Invention
The present invention relates to a video coding apparatus, a video coding method and a video decoding apparatus for reducing pulsing artifacts which occur, at the time when various multimedia data are coded, and especially at the time when intra-picture coding is used for coding video.
(2) Description of the Related Art
In general, most video coding algorithms, such as the ISO/IEC 14496-2 Part 10 International Standard, code an intra-picture, which is also known as a key picture, at the start of each group of consecutive inter-predicted pictures. An intra-coded picture is a self-contained picture which does not depend on neighbouring pictures to be decoded. The intra-coded picture is sometimes used as an access point for a video decoder to independently decode a specific group of pictures (GOP) which includes the intra-picture and the inter-predicted pictures following the intra-coded picture without causing the decoder to decode pictures prior to the intra-coded picture in the compressed video stream. A coded video sequence is sometimes made up of several groups of pictures to facilitate random access and decoding of a portion of the coded video sequence.
The “pulsing” effect is an artifact that occurs when the coding quality of an intra-picture is different from that of the inter-pictures coded prior to the intra-picture. In another words, this artifact occurs at the transition part of one group of pictures to another group of pictures. For a coded video sequence that is made up of groups of pictures, this artifact appears like a “pulse” at every group of pictures interval due to a different intra-picture quality. The pulsing effect is more noticeable at a region of a picture with small motion activities especially in the case where the video sequence is coded at very high compression rate. FIG. 1 shows some examples of where pulsing artifacts occur in a video sequence. As shown in the figure, pulsing artifacts occurs at the GOP boundary between two different GOPs.
To reduce this pulsing effect, one prior art is to use a rate control algorithm to control the quality of the intra-picture. This pulsing effect is usually caused when the intra-picture is over compressed. Thus, one method to reduce this pulsing effect is to reduce the compression ratio for the intra-picture while increasing the compression ratio for the inter-pictures.
In addition, Patent Reference 1 (Japanese unexamined Patent Application Publication No. 9-130648) discloses a technique for reducing artifacts by executing filtering in a temporal direction after an image, including artifacts such as pulsing which occur in coding, is decoded.
However, the problem with the prior art is that an increase in the compression ratio reduces the quality of the inter-pictures, resulting in the increase of other artifacts such as blocking and ringing. In other words, the problem with the prior art is that it is impossible to improve the visual quality of pictures because other artifacts increase even in the case where pulsing artifacts decrease.
Pulsing effect is a noticeable artifact which occurs in a video compressed at a high compression ratio using a video coding algorithm such as the ISO/IEC 14496-2 Part 10 International Standard. In order to improve the picture quality of the compressed video, this artifact needs to be reduced either prior to the coding or after the decoding of the video. The problem with the prior art, that reduces the compression ratio of the intra-picture to reduce this pulsing effect, is that in order to maintain a certain overall compression ratio within the group of pictures, the quality of the inter-pictures is reduced due to a higher compression ratio. Thus, although the pulsing effect is reduced at the boundaries between the groups of pictures, the prior art introduces more artifacts to the inter-pictures within the group of pictures.
In addition, in the technique disclosed in Patent Reference 1, pulsing artifacts cannot be reduced until decoding is performed. Since coded image data contains pulsing artifacts, there is a problem that pulsing artifacts cannot be reduced in the case where the image data is decoded by another decoding apparatus in which the technique disclosed in Patent Reference 1 is not mounted.
In view of this, the present invention aims at providing a video coding apparatus and method for reducing pulsing artifacts which occur especially at transition parts between groups of pictures and improving the visual quality of the pictures. Further, the present invention aims at providing a video coding apparatus and method capable of reducing pulsing artifacts in coding.